FRI475 The Impact Of Suppression Of Androgen Signaling On Thyroid Hormone Homeostasis In Prostate Cancer Patients

Disclosure: K. David: None. G. Devos: None. W. Devlies: None. N. Narinx: None. L. Antonio: None. L. Deboel: None. D. Schollaert: None. D.M. Vanderschueren: None. F.A. Claessens: None. S. Joniau: None. B. Decallonne: None. Next to the classical androgen deprivation therapy (ADT) with GnRH-(ant)agonists, androgen receptor signaling inhibitors (ARSIs) such as apalutamide (APA) have become available for the treatment of prostate cancer (PC) patients in the last years. Hypothyroidism was more frequent in APA-treated patients compared to placebo in the SPARTAN trial (NCT01946204) (8.1% vs 2%). We collected serum of PC patients included in a RCT (ARNEO; NCT03080116) before and after 12 weeks of neo-adjuvant ADT (GnRH-antagonist degarelix (DGX) with APA or placebo). We aimed to explore changes in thyroid homeostasis under deep androgen suppression with combined GnRH-antagonist and ARSI. Of the 88 patients ending the trial, 1 was excluded because of history of hypothyroidism for which he received levothyroxine. 43 and 44 patients were included in the DGX+placebo and DGX+APA group, respectively. Mean age was 66.3y. Sex steroid levels measured by LC-MS/MS decreased significantly to castrate range with median testosterone 10.5 ng/dL and estradiol 3.6 ng/L. TSH levels increased in both treatment groups, but more pronounced in the DGX+APA group (DGX+placebo before: 1.18 mIU/L and after: 1.33 mIU/L, p 0.0351 vs. DGX+APA before: 1.07 mIU/L and after: 1.87 mIU/L, p <0.0001; difference between groups after treatment: p <0.0001). Free T4 and total T4 levels decreased in the DGX+APA group, but remained unaltered in the DGX+placebo group (free T4 after: 16.53 vs 13.87 pmol/L, p<0.0001 and total T4 after: 8.77 vs. 7.78 µg/dL, p 0.0055). Total T3 and free T3 increased in the DGX+APA group, and again did not differ after treatment in the DGX+placebo group. (free T3 after: 5.03 vs 5.41 pmol/L, p 0.0066 and total T3 after: 132.30 vs 149.50 ng/dL, p 0.0003). As a result, the total and free T3/T4 ratio increased in the DGX+APA group with 32% and 35%, respectively. Reverse T3 decreased in both treatment arms, but greater in the DGX+APA group (19.55 vs 14.20 ng/dL, p 0.0157). Thyroxine binding globulin (TBG) increased in both treatment arms, with 6.9% and 8.4%, respectively. In conclusion, interference with androgen signaling alters thyroid homeostasis in PC patients, and is most pronounced under deep androgen suppression with combined GnRH-antagonist and ARSI therapy. The increase in T3 and decrease in rT3 may indicate additional effects of ARSI on deiodinase activity. Longer follow-up trials are needed to confirm these findings and assess the clinical implications of these changes in thyroid homeostasis. Presentation: Friday, June 16, 2023